The present invention relates to a pull method of producing a single crystal from such a melt as metal, semiconductive material or oxide under a stabilized condition inhibiting the formation of minute faults or dislocations, and also relates to an apparatus useful for said pull method.
Czochralski method is a representative method for the growth of a single crystal from a melt.
Czochralski method uses a crucible 2 provided in a closed chamber 1 as shown in FIG. 1. The crucible 2 is supported by a supportor 3 so that the crucible 2 is capable of rotation and/or vertical motion. There are concentrically disposed a heater 4 and a heat insulator 5 around the crucible 2. A raw material is received in the crucible 2 and intensively heated by the heater 4 to prepare a melt 6. The melt 6 is held at a temperature suitable for the growth of a single crystal. When a seed crystal 7 being hung from a rotary winder 10 through a wire 9 is brought into contact with the melt 6, a single crystal 8 grows on the seed crystal 7 so that the crystalline orientation of the seed crystal 7 is transferred to that of the growing crystal 8. The seed crystal 7 is then rotatingly pulled up in response to the growth of the single crystal 8. The crucible 2 is descendingly rotated by the rotating motion of the support 3, too. The obtained single crystal 8 is sheared to wafers to be used as substrates for semiconductor devices.
By the way, semiconductor substrates of higher quality are demanded, as semiconductor devices are integrated with higher density. In order to obtain a semiconductor substrate of high quality, it is necessary to inhibit dislocations or cystalline faults which would be included in the single crystal. On the other hand, the production :of substrates for semiconductor devices with high productivity requires the improvement which enables the growth of a single crystal larger in diameter.
According to Czochlarski method, a single crystal is pulled up from a melt held at a high temperature, so that the quality of the obtained single crystal is significantly affected by the conditions of the melt near the interface of crystal growth.
There are clusters, i.e. agglomerates different in minute structures, in the melt which changes physical properties to larger extents. The minute structures are varied in response to the thermal hysteresis of the melt. In case where a melt just after prepared by melting a polycrystalline raw material is held at a temperature slightly higher than its melting point, there remain a lot of clusters having covalent bonds which have transferred the crystalline structure of the raw material. In addition, there are irregular structures in the interface of crystal growth. When a single crystal is let grow from such a heterogeneous melt, the regular rearrangement of molten atoms on a seed crystal is impeded by the covalent bond of the clusters. Minute faults and dislocations originated in the irregular structure of the melt are likely to be included in the single crystal pulled up from such the melt. Consequently, single crystals of stabilized quality can not be produced at a high yield.
In principle, the melt can be conditioned to a homogeneous state suitable for the growth of a single crystal free from minute faults and dislocations, by heating the melt at a temperature above the critical point at which irregular structures in the melt disappear, or by controlling the temperature of the melt with high accuracy. However, the melt is inevitably heated at a temperature, e.g. 1440.degree.-1460.degree. C., sufficiently higher than the critical point for a long time, e.g. 3-4 hrs., since there have been known neither an accurate critical point nor the influence of temperature fluctuation on the minute structures of the melt. The heating time is experimentally determined taking into consideration various factors such as the material of the melt, the capacity of the crucible and the power of a heater.
The excessive heating requires a long time before the initiation of operations for pulling up the single crystal in addition to the consumption of a large quantity of thermal energy. As a result, the single crystals are not produced with high productivity at a low cost.
These disadvantages can be overcome, if it is possible to make short the period from the melting of the raw material to the initiation of pulling up the single crystal. In this regard, it is necessary to accurately detect the physical properties of the melt and the fluctuation of the physical properties in a short time. The obtained data will be effective for judging whether the melt is stable or unstable. We have proposed a new density detector as disclosed Japanese Patent Application 4-317900. The density of a melt can be accurately detected without errors derived from buoyancy, by using the density detector. During the investigation of various melts using the proposed density detector, we have found the phenomena that the melt prepared by melting a polycrystalline raw material has the tendency to lower its density as the rising of the temperature and that there is the reflection point where the changing rate of the density with respect to the temperature of the melt apparently differs from each other during the temperature falling of the melt.